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Ballpark calculating small PP-OPT primary impedance?

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Is this a valid rule of thumb?...

From the data sheet I know the Ra (dynamic plate resistance) of a pentode is 11K and I want to run a pair in PP. Do I just multiply 11K * 1.4 to obtain a ballpark impedance load for the OPT primary anode-to-anode? So in this case I would need to go find a PP OPT in my junk box that has approximately 15.4K primary? Is there a better calculation if the data sheet has no explicit figures for PP operation (typical for non-beam tubes)? (this is not for a beam tube, the plate dissipation is only a few watts like in a kids record player, so a small radio OPT purpose is fine).

Also if I do find a a couple of junk-box OPT's both over and under 15.4K (by lets say 20%) is it "better" to use the OPT that is over or under 15.4K?

Hammond makes their 125 series universal OPT's where you select your speaker ohms, (that sets up the reflected impedance), then you pick the closest set of primary taps. If I use a Hammond OPT it would give me a choice of either 12,800 or 17,600 primaries. In this scenario which would be the best, to go over or under my ballpark 15.4K?
 
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Forget all these "Rules of Thumb".....non-sense.. Those are the quickest ways of making a wreck of a design...
When dealing with pentodes...the key is to use the proper curves.... Make sure the screen voltage on the curve sheet is same as application....Most important curve is the 0-Bias curve... Plot the load-line from your B+ to just a bit to the right of the knee of the 0-Bias curve... The load-Line will be 1/4 the Plate-Load...in most cases..
What tube and what are the intended voltages ??
 
Thanks, I'll try to get through that!

I have about 60 NOS ECL80 / 6AB8 Mullards in original boxes that I got at an estate sale, they are an odd kind of tube. Common cathode, triode/pentode. They were used a lot in one-tube kids phonographs in the 50's single ended, or as PP TV audio in small TV's (Europe). I was hoping to make a few small cheap PP amps out of them in small enclosures then find a use for the assembled mini-amps later. Nothing audiophile or critical. Old portable TV's sounded "nice" back then as I recall (I'm 65). Better sounding than the tinny membrane speakers on the small flat screen monitors of today. Maybe try a PP headphone AMP. Or some weird guitar pedal. Something like that, to have some fun bread boarding up a few of these Mullards.

Even doing a full-on load line analysis, I'm still going to be up against choosing a compromise winding on a small hammond 125 or other "universal" OPT at the end. I was thinking 200V, all cathodes tied together (literally) because both sections use same cathode, cathode biased all at once on one wire, both tubes all sections.

Two tubes, the pentode sections PP and the other two triodes (one from each tube) as follows triode #1 as input gain and the triode #2 as a concertina phase inverter. No feedback, cheap universal OPT.
 
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PRR

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...the Ra (dynamic plate resistance) of a pentode is 11K and I want to run a pair in PP....

A pentode has TWO "Ra". Above the knee (many K Ohms) and below the knee (hundreds of Ohms).

The "ideal" load is roughly the geometric mean, but really you do not want to get too close to either limit.

However the internal (above knee) Ra of 6AB8 is NOT "11K"!! Ra is 150K!

However this 11K IS the suggested load for SE. So you have a maker-suggested value. For simple class A P-P you simply double it. 22K. At 200V, expect a hair more than double the 1.4W of SE because P-P cancels much of the high THD of SE. Say 3 Watts at <10% THD.
 
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I have done a bit of research on ECL80 applications, because I also have 50+ tubes from a old repair shop. This tube was common and inexpensive; many repair shops bought full cartons of them. There is a official Mullard application note for a PP amplifier at 200V.

It was published on Wireless World magazine in 1950. It has apparently been actually built by several people and it should work satisfactorily, altough the primary focus of the designer was cost reduction and the circuit does not seems to be the best possible way to use this tube. I believe that a standard 110+110V to 6V power transformer could be used, maybe a small toroidal one.

A derivative of this circuit has been implemented on several Ferguson TV sets and published again several times, from the '80 onwards; I attach an example from Television magazine, another example is currently on the Instructable web site. This tube is one of the very first with multiple elements on noval base, it predates most double triode implementations and it does have a 6.3V 300mA filament.

A different option, with 2 tubes per channel, is a parallel single ended amplifier. I believe that a standard 5k 40mA 3W transformer could be used.
 

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How does that work, concertina with cathode common to 3 other units?

I was wrong about that and just assumed it was a typical concertina. In looking at the few commercial diagrams I can find (only two so far). The cathodes are one big string to the cathode resistor which is bypassed. The phase inversion is still tapped off the anode but the other side is taken from the grid prior to the stopper resistor instead of the cathode like a concertina. I don't know what its called, it's one tube doing the inversion so I thought it was a concertina until your correction. A grid/anode concertina? :)
 
I have done a bit of research on ECL80 applications, because I also have 50+ tubes from a old repair shop. A different option, with 2 tubes per channel, is a parallel single ended amplifier. I believe that a standard 5k 40mA 3W transformer could be used.

Nice, thanks! I was looking at the Mullard bulletin from a post on that same forum.

Mullard Valves, Tubes & Circuits and Frame Grid Valves

Parallel SE, mmmm
 
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I've found a good explanation of this circuit on Radio Constructor magazine (Volume 10, number 9, april 1957, page 588). The Radio Constructor implementation does use a live chassis, so I haven't attached the schematic to my previous post. The single ended ECL80 circuit has been published on the following month, page 704.
 
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